The present invention relates to an electric steering apparatus which includes an electric motor for applying torque to a steering system through a speed reduction mechanism, and a rotation detector for detecting a rotation angle of the rotational shaft of the electric motor.
Electric power steering apparatus for assisting the driver's steering operation is known as an electric steering apparatus which applies a driving force of the electric motor to the steering shaft through the speed reduction mechanism. As shown in FIG. 4, Japanese Laid-open Patent Application No. 2002-331946 (paragraph numbers 0032 and 0033, FIG. 4) discloses such an electric power steering apparatus with a brushless motor 100.
In this electric power steering apparatus 200 using the brushless motor 100, a drawback in the brushless motor 100, such as voltage drop between brush and commutator due to wear of the brush by repeated use over extended time period, can be overcome, and thus a stable steering assist force is advantageously obtained without a decrease or variation in output of the motor. Further, because the brushless motor 100 is smaller in moment of inertia than the brush motor, excellent steering feeling can be obtained during high-speed straight advance movement of the vehicle or upon turning the steering wheel.
However, such a brushless motor 100 requires an accurate control of electric current supply in accordance with the rotation of the rotational shaft 101 because the brushless motor 100 does not stop the rotation or decrease the rotational speed by the friction between the brush and the commutator. Therefore, in the brushless motor 100, it is necessary to provide a rotation detector 300 for detecting the rotation speed, etc. of the rotational shaft 101, and a current detector (not shown) for detecting electric current flowing through the brushless motor 100, so as to control the brushless motor 100 based on the output signals from these detectors.
However, in this brushless motor 100, if the motor rotation signal from the rotation detector 300 contains a noise, the output of the motor varies and a smooth steering feeling may not be obtained. For example, if the rotation detector 300 is subject to dust, rust due to dew condensation or a deformation by an external force, a noise arises on the motor rotation signal and a smooth steering feeling will be lost. Therefore, the rotation detector 300 is covered by a cover C so that the affect of rust and the like to the rotation detector 300 can be prevented and the motor rotation signal from the rotation detector 300 can be outputted in a stable manner to thereby achieve a smooth steering feeling.
In the conventional electric power steering apparatus, because a separate cover C is provided other than the motor housing 400 for accommodating the brushless motor 100, the manufacturing cost increases accordingly. The rotation detector 300 may be arranged in the motor housing 400 between the bearings 500, 501 which support the rotational shaft 101 of the brushless motor 100 in a manner adjacent to the brushless motor 100. However, with increasing the distance (center distance) between the bearings 500, 501, the rotational shaft 101 has to be extended accordingly. This leads to an increase in size of the brushless motor 100 and a decrease in rigidity of the rotational shaft 101, which is likely to cause vibrations. Increasing the diameter of the rotational shaft 101 to prevent such vibrations makes the weight of the electric power steering apparatus heavier, which decreases the steering feeling. As shown in FIG. 5, relations between vibration, center distance and shaft diameter (size of the shaft) of the rotational shaft 101 are such that more vibration occurs as the center distance becomes greater or as the shaft diameter becomes smaller. On the contrary, less vibration occurs as the center distance becomes smaller or as the shaft diameter becomes greater.
In view of the above, the present invention seeks to provide an electric steering apparatus, which prevents variation of the output from the rotation detector, and without increasing the number of parts which decreases cost while improves steering feeling.